RFC 0959
File Transfer Protocol
Pages: 69
Internet Standard: 9
→ Errata
Obsoletes: 0765
Updated by: 2228 2640 2773 3659 5797 7151
Internet Standard: 9
→ Errata
Obsoletes: 0765
Updated by: 2228 2640 2773 3659 5797 7151
Part 2 of 3 – Pages 18 to 43
3.2. ESTABLISHING DATA CONNECTIONS The mechanics of transferring data consists of setting up the data connection to the appropriate ports and choosing the parameters for transfer. Both the user and the server-DTPs have a default data port. The user-process default data port is the same as the control connection port (i.e., U). The server-process default data port is the port adjacent to the control connection port (i.e., L-1). The transfer byte size is 8-bit bytes. This byte size is relevant only for the actual transfer of the data; it has no bearing on representation of the data within a host's file system. The passive data transfer process (this may be a user-DTP or a second server-DTP) shall "listen" on the data port prior to sending a transfer request command. The FTP request command determines the direction of the data transfer. The server, upon receiving the transfer request, will initiate the data connection to the port. When the connection is established, the data transfer begins between DTP's, and the server-PI sends a confirming reply to the user-PI. Every FTP implementation must support the use of the default data ports, and only the USER-PI can initiate a change to non-default ports. It is possible for the user to specify an alternate data port by use of the PORT command. The user may want a file dumped on a TAC line printer or retrieved from a third party host. In the latter case, the user-PI sets up control connections with both server-PI's. One server is then told (by an FTP command) to "listen" for a connection which the other will initiate. The user-PI sends one server-PI a PORT command indicating the data port of the other. Finally, both are sent the appropriate transfer commands. The exact sequence of commands and replies sent between the user-controller and the servers is defined in the Section on FTP Replies. In general, it is the server's responsibility to maintain the data connection--to initiate it and to close it. The exception to this
is when the user-DTP is sending the data in a transfer mode that
requires the connection to be closed to indicate EOF. The server
MUST close the data connection under the following conditions:
1. The server has completed sending data in a transfer mode
that requires a close to indicate EOF.
2. The server receives an ABORT command from the user.
3. The port specification is changed by a command from the
user.
4. The control connection is closed legally or otherwise.
5. An irrecoverable error condition occurs.
Otherwise the close is a server option, the exercise of which the
server must indicate to the user-process by either a 250 or 226
reply only.
3.3. DATA CONNECTION MANAGEMENT
Default Data Connection Ports: All FTP implementations must
support use of the default data connection ports, and only the
User-PI may initiate the use of non-default ports.
Negotiating Non-Default Data Ports: The User-PI may specify a
non-default user side data port with the PORT command. The
User-PI may request the server side to identify a non-default
server side data port with the PASV command. Since a connection
is defined by the pair of addresses, either of these actions is
enough to get a different data connection, still it is permitted
to do both commands to use new ports on both ends of the data
connection.
Reuse of the Data Connection: When using the stream mode of data
transfer the end of the file must be indicated by closing the
connection. This causes a problem if multiple files are to be
transfered in the session, due to need for TCP to hold the
connection record for a time out period to guarantee the reliable
communication. Thus the connection can not be reopened at once.
There are two solutions to this problem. The first is to
negotiate a non-default port. The second is to use another
transfer mode.
A comment on transfer modes. The stream transfer mode is
inherently unreliable, since one can not determine if the
connection closed prematurely or not. The other transfer modes
(Block, Compressed) do not close the connection to indicate the
end of file. They have enough FTP encoding that the data
connection can be parsed to determine the end of the file.
Thus using these modes one can leave the data connection open
for multiple file transfers.
3.4. TRANSMISSION MODES
The next consideration in transferring data is choosing the
appropriate transmission mode. There are three modes: one which
formats the data and allows for restart procedures; one which also
compresses the data for efficient transfer; and one which passes
the data with little or no processing. In this last case the mode
interacts with the structure attribute to determine the type of
processing. In the compressed mode, the representation type
determines the filler byte.
All data transfers must be completed with an end-of-file (EOF)
which may be explicitly stated or implied by the closing of the
data connection. For files with record structure, all the
end-of-record markers (EOR) are explicit, including the final one.
For files transmitted in page structure a "last-page" page type is
used.
NOTE: In the rest of this section, byte means "transfer byte"
except where explicitly stated otherwise.
For the purpose of standardized transfer, the sending host will
translate its internal end of line or end of record denotation
into the representation prescribed by the transfer mode and file
structure, and the receiving host will perform the inverse
translation to its internal denotation. An IBM Mainframe record
count field may not be recognized at another host, so the
end-of-record information may be transferred as a two byte control
code in Stream mode or as a flagged bit in a Block or Compressed
mode descriptor. End-of-line in an ASCII or EBCDIC file with no
record structure should be indicated by <CRLF> or <NL>,
respectively. Since these transformations imply extra work for
some systems, identical systems transferring non-record structured
text files might wish to use a binary representation and stream
mode for the transfer.
The following transmission modes are defined in FTP:
3.4.1. STREAM MODE
The data is transmitted as a stream of bytes. There is no
restriction on the representation type used; record structures
are allowed.
In a record structured file EOR and EOF will each be indicated
by a two-byte control code. The first byte of the control code
will be all ones, the escape character. The second byte will
have the low order bit on and zeros elsewhere for EOR and the
second low order bit on for EOF; that is, the byte will have
value 1 for EOR and value 2 for EOF. EOR and EOF may be
indicated together on the last byte transmitted by turning both
low order bits on (i.e., the value 3). If a byte of all ones
was intended to be sent as data, it should be repeated in the
second byte of the control code.
If the structure is a file structure, the EOF is indicated by
the sending host closing the data connection and all bytes are
data bytes.
3.4.2. BLOCK MODE
The file is transmitted as a series of data blocks preceded by
one or more header bytes. The header bytes contain a count
field, and descriptor code. The count field indicates the
total length of the data block in bytes, thus marking the
beginning of the next data block (there are no filler bits).
The descriptor code defines: last block in the file (EOF) last
block in the record (EOR), restart marker (see the Section on
Error Recovery and Restart) or suspect data (i.e., the data
being transferred is suspected of errors and is not reliable).
This last code is NOT intended for error control within FTP.
It is motivated by the desire of sites exchanging certain types
of data (e.g., seismic or weather data) to send and receive all
the data despite local errors (such as "magnetic tape read
errors"), but to indicate in the transmission that certain
portions are suspect). Record structures are allowed in this
mode, and any representation type may be used.
The header consists of the three bytes. Of the 24 bits of
header information, the 16 low order bits shall represent byte
count, and the 8 high order bits shall represent descriptor
codes as shown below.
Block Header
+----------------+----------------+----------------+
| Descriptor | Byte Count |
| 8 bits | 16 bits |
+----------------+----------------+----------------+
The descriptor codes are indicated by bit flags in the
descriptor byte. Four codes have been assigned, where each
code number is the decimal value of the corresponding bit in
the byte.
Code Meaning
128 End of data block is EOR
64 End of data block is EOF
32 Suspected errors in data block
16 Data block is a restart marker
With this encoding, more than one descriptor coded condition
may exist for a particular block. As many bits as necessary
may be flagged.
The restart marker is embedded in the data stream as an
integral number of 8-bit bytes representing printable
characters in the language being used over the control
connection (e.g., default--NVT-ASCII). <SP> (Space, in the
appropriate language) must not be used WITHIN a restart marker.
For example, to transmit a six-character marker, the following
would be sent:
+--------+--------+--------+
|Descrptr| Byte count |
|code= 16| = 6 |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
+--------+--------+--------+
| Marker | Marker | Marker |
| 8 bits | 8 bits | 8 bits |
+--------+--------+--------+
3.4.3. COMPRESSED MODE There are three kinds of information to be sent: regular data, sent in a byte string; compressed data, consisting of replications or filler; and control information, sent in a two-byte escape sequence. If n>0 bytes (up to 127) of regular data are sent, these n bytes are preceded by a byte with the left-most bit set to 0 and the right-most 7 bits containing the number n. Byte string: 1 7 8 8 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |0| n | | d(1) | ... | d(n) | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ ^ ^ |---n bytes---| of data String of n data bytes d(1),..., d(n) Count n must be positive. To compress a string of n replications of the data byte d, the following 2 bytes are sent: Replicated Byte: 2 6 8 +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ |1 0| n | | d | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ A string of n filler bytes can be compressed into a single byte, where the filler byte varies with the representation type. If the type is ASCII or EBCDIC the filler byte is <SP> (Space, ASCII code 32, EBCDIC code 64). If the type is Image or Local byte the filler is a zero byte. Filler String: 2 6 +-+-+-+-+-+-+-+-+ |1 1| n | +-+-+-+-+-+-+-+-+ The escape sequence is a double byte, the first of which is the
escape byte (all zeros) and the second of which contains
descriptor codes as defined in Block mode. The descriptor
codes have the same meaning as in Block mode and apply to the
succeeding string of bytes.
Compressed mode is useful for obtaining increased bandwidth on
very large network transmissions at a little extra CPU cost.
It can be most effectively used to reduce the size of printer
files such as those generated by RJE hosts.
3.5. ERROR RECOVERY AND RESTART
There is no provision for detecting bits lost or scrambled in data
transfer; this level of error control is handled by the TCP.
However, a restart procedure is provided to protect users from
gross system failures (including failures of a host, an
FTP-process, or the underlying network).
The restart procedure is defined only for the block and compressed
modes of data transfer. It requires the sender of data to insert
a special marker code in the data stream with some marker
information. The marker information has meaning only to the
sender, but must consist of printable characters in the default or
negotiated language of the control connection (ASCII or EBCDIC).
The marker could represent a bit-count, a record-count, or any
other information by which a system may identify a data
checkpoint. The receiver of data, if it implements the restart
procedure, would then mark the corresponding position of this
marker in the receiving system, and return this information to the
user.
In the event of a system failure, the user can restart the data
transfer by identifying the marker point with the FTP restart
procedure. The following example illustrates the use of the
restart procedure.
The sender of the data inserts an appropriate marker block in the
data stream at a convenient point. The receiving host marks the
corresponding data point in its file system and conveys the last
known sender and receiver marker information to the user, either
directly or over the control connection in a 110 reply (depending
on who is the sender). In the event of a system failure, the user
or controller process restarts the server at the last server
marker by sending a restart command with server's marker code as
its argument. The restart command is transmitted over the control
connection and is immediately followed by the command (such as
RETR, STOR or LIST) which was being executed when the system
failure occurred.
4. FILE TRANSFER FUNCTIONS
The communication channel from the user-PI to the server-PI is
established as a TCP connection from the user to the standard server
port. The user protocol interpreter is responsible for sending FTP
commands and interpreting the replies received; the server-PI
interprets commands, sends replies and directs its DTP to set up the
data connection and transfer the data. If the second party to the
data transfer (the passive transfer process) is the user-DTP, then it
is governed through the internal protocol of the user-FTP host; if it
is a second server-DTP, then it is governed by its PI on command from
the user-PI. The FTP replies are discussed in the next section. In
the description of a few of the commands in this section, it is
helpful to be explicit about the possible replies.
4.1. FTP COMMANDS
4.1.1. ACCESS CONTROL COMMANDS
The following commands specify access control identifiers
(command codes are shown in parentheses).
USER NAME (USER)
The argument field is a Telnet string identifying the user.
The user identification is that which is required by the
server for access to its file system. This command will
normally be the first command transmitted by the user after
the control connections are made (some servers may require
this). Additional identification information in the form of
a password and/or an account command may also be required by
some servers. Servers may allow a new USER command to be
entered at any point in order to change the access control
and/or accounting information. This has the effect of
flushing any user, password, and account information already
supplied and beginning the login sequence again. All
transfer parameters are unchanged and any file transfer in
progress is completed under the old access control
parameters.
PASSWORD (PASS)
The argument field is a Telnet string specifying the user's
password. This command must be immediately preceded by the
user name command, and, for some sites, completes the user's
identification for access control. Since password
information is quite sensitive, it is desirable in general
to "mask" it or suppress typeout. It appears that the
server has no foolproof way to achieve this. It is
therefore the responsibility of the user-FTP process to hide
the sensitive password information.
ACCOUNT (ACCT)
The argument field is a Telnet string identifying the user's
account. The command is not necessarily related to the USER
command, as some sites may require an account for login and
others only for specific access, such as storing files. In
the latter case the command may arrive at any time.
There are reply codes to differentiate these cases for the
automation: when account information is required for login,
the response to a successful PASSword command is reply code
332. On the other hand, if account information is NOT
required for login, the reply to a successful PASSword
command is 230; and if the account information is needed for
a command issued later in the dialogue, the server should
return a 332 or 532 reply depending on whether it stores
(pending receipt of the ACCounT command) or discards the
command, respectively.
CHANGE WORKING DIRECTORY (CWD)
This command allows the user to work with a different
directory or dataset for file storage or retrieval without
altering his login or accounting information. Transfer
parameters are similarly unchanged. The argument is a
pathname specifying a directory or other system dependent
file group designator.
CHANGE TO PARENT DIRECTORY (CDUP)
This command is a special case of CWD, and is included to
simplify the implementation of programs for transferring
directory trees between operating systems having different
syntaxes for naming the parent directory. The reply codes
shall be identical to the reply codes of CWD. See
Appendix II for further details.
STRUCTURE MOUNT (SMNT)
This command allows the user to mount a different file
system data structure without altering his login or
accounting information. Transfer parameters are similarly
unchanged. The argument is a pathname specifying a
directory or other system dependent file group designator.
REINITIALIZE (REIN)
This command terminates a USER, flushing all I/O and account
information, except to allow any transfer in progress to be
completed. All parameters are reset to the default settings
and the control connection is left open. This is identical
to the state in which a user finds himself immediately after
the control connection is opened. A USER command may be
expected to follow.
LOGOUT (QUIT)
This command terminates a USER and if file transfer is not
in progress, the server closes the control connection. If
file transfer is in progress, the connection will remain
open for result response and the server will then close it.
If the user-process is transferring files for several USERs
but does not wish to close and then reopen connections for
each, then the REIN command should be used instead of QUIT.
An unexpected close on the control connection will cause the
server to take the effective action of an abort (ABOR) and a
logout (QUIT).
4.1.2. TRANSFER PARAMETER COMMANDS
All data transfer parameters have default values, and the
commands specifying data transfer parameters are required only
if the default parameter values are to be changed. The default
value is the last specified value, or if no value has been
specified, the standard default value is as stated here. This
implies that the server must "remember" the applicable default
values. The commands may be in any order except that they must
precede the FTP service request. The following commands
specify data transfer parameters:
DATA PORT (PORT)
The argument is a HOST-PORT specification for the data port
to be used in data connection. There are defaults for both
the user and server data ports, and under normal
circumstances this command and its reply are not needed. If
this command is used, the argument is the concatenation of a
32-bit internet host address and a 16-bit TCP port address.
This address information is broken into 8-bit fields and the
value of each field is transmitted as a decimal number (in
character string representation). The fields are separated
by commas. A port command would be:
PORT h1,h2,h3,h4,p1,p2
where h1 is the high order 8 bits of the internet host
address.
PASSIVE (PASV)
This command requests the server-DTP to "listen" on a data
port (which is not its default data port) and to wait for a
connection rather than initiate one upon receipt of a
transfer command. The response to this command includes the
host and port address this server is listening on.
REPRESENTATION TYPE (TYPE)
The argument specifies the representation type as described
in the Section on Data Representation and Storage. Several
types take a second parameter. The first parameter is
denoted by a single Telnet character, as is the second
Format parameter for ASCII and EBCDIC; the second parameter
for local byte is a decimal integer to indicate Bytesize.
The parameters are separated by a <SP> (Space, ASCII code
32).
The following codes are assigned for type:
\ /
A - ASCII | | N - Non-print
|-><-| T - Telnet format effectors
E - EBCDIC| | C - Carriage Control (ASA)
/ \
I - Image
L <byte size> - Local byte Byte size
The default representation type is ASCII Non-print. If the
Format parameter is changed, and later just the first
argument is changed, Format then returns to the Non-print
default.
FILE STRUCTURE (STRU)
The argument is a single Telnet character code specifying
file structure described in the Section on Data
Representation and Storage.
The following codes are assigned for structure:
F - File (no record structure)
R - Record structure
P - Page structure
The default structure is File.
TRANSFER MODE (MODE)
The argument is a single Telnet character code specifying
the data transfer modes described in the Section on
Transmission Modes.
The following codes are assigned for transfer modes:
S - Stream
B - Block
C - Compressed
The default transfer mode is Stream.
4.1.3. FTP SERVICE COMMANDS
The FTP service commands define the file transfer or the file
system function requested by the user. The argument of an FTP
service command will normally be a pathname. The syntax of
pathnames must conform to server site conventions (with
standard defaults applicable), and the language conventions of
the control connection. The suggested default handling is to
use the last specified device, directory or file name, or the
standard default defined for local users. The commands may be
in any order except that a "rename from" command must be
followed by a "rename to" command and the restart command must
be followed by the interrupted service command (e.g., STOR or
RETR). The data, when transferred in response to FTP service
commands, shall always be sent over the data connection, except
for certain informative replies. The following commands
specify FTP service requests:
RETRIEVE (RETR)
This command causes the server-DTP to transfer a copy of the
file, specified in the pathname, to the server- or user-DTP
at the other end of the data connection. The status and
contents of the file at the server site shall be unaffected.
STORE (STOR)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data as
a file at the server site. If the file specified in the
pathname exists at the server site, then its contents shall
be replaced by the data being transferred. A new file is
created at the server site if the file specified in the
pathname does not already exist.
STORE UNIQUE (STOU)
This command behaves like STOR except that the resultant
file is to be created in the current directory under a name
unique to that directory. The 250 Transfer Started response
must include the name generated.
APPEND (with create) (APPE)
This command causes the server-DTP to accept the data
transferred via the data connection and to store the data in
a file at the server site. If the file specified in the
pathname exists at the server site, then the data shall be
appended to that file; otherwise the file specified in the
pathname shall be created at the server site.
ALLOCATE (ALLO)
This command may be required by some servers to reserve
sufficient storage to accommodate the new file to be
transferred. The argument shall be a decimal integer
representing the number of bytes (using the logical byte
size) of storage to be reserved for the file. For files
sent with record or page structure a maximum record or page
size (in logical bytes) might also be necessary; this is
indicated by a decimal integer in a second argument field of
the command. This second argument is optional, but when
present should be separated from the first by the three
Telnet characters <SP> R <SP>. This command shall be
followed by a STORe or APPEnd command. The ALLO command
should be treated as a NOOP (no operation) by those servers
which do not require that the maximum size of the file be
declared beforehand, and those servers interested in only
the maximum record or page size should accept a dummy value
in the first argument and ignore it.
RESTART (REST)
The argument field represents the server marker at which
file transfer is to be restarted. This command does not
cause file transfer but skips over the file to the specified
data checkpoint. This command shall be immediately followed
by the appropriate FTP service command which shall cause
file transfer to resume.
RENAME FROM (RNFR)
This command specifies the old pathname of the file which is
to be renamed. This command must be immediately followed by
a "rename to" command specifying the new file pathname.
RENAME TO (RNTO)
This command specifies the new pathname of the file
specified in the immediately preceding "rename from"
command. Together the two commands cause a file to be
renamed.
ABORT (ABOR)
This command tells the server to abort the previous FTP
service command and any associated transfer of data. The
abort command may require "special action", as discussed in
the Section on FTP Commands, to force recognition by the
server. No action is to be taken if the previous command
has been completed (including data transfer). The control
connection is not to be closed by the server, but the data
connection must be closed.
There are two cases for the server upon receipt of this
command: (1) the FTP service command was already completed,
or (2) the FTP service command is still in progress.
In the first case, the server closes the data connection
(if it is open) and responds with a 226 reply, indicating
that the abort command was successfully processed.
In the second case, the server aborts the FTP service in
progress and closes the data connection, returning a 426
reply to indicate that the service request terminated
abnormally. The server then sends a 226 reply,
indicating that the abort command was successfully
processed.
DELETE (DELE)
This command causes the file specified in the pathname to be
deleted at the server site. If an extra level of protection
is desired (such as the query, "Do you really wish to
delete?"), it should be provided by the user-FTP process.
REMOVE DIRECTORY (RMD)
This command causes the directory specified in the pathname
to be removed as a directory (if the pathname is absolute)
or as a subdirectory of the current working directory (if
the pathname is relative). See Appendix II.
MAKE DIRECTORY (MKD)
This command causes the directory specified in the pathname
to be created as a directory (if the pathname is absolute)
or as a subdirectory of the current working directory (if
the pathname is relative). See Appendix II.
PRINT WORKING DIRECTORY (PWD)
This command causes the name of the current working
directory to be returned in the reply. See Appendix II.
LIST (LIST)
This command causes a list to be sent from the server to the
passive DTP. If the pathname specifies a directory or other
group of files, the server should transfer a list of files
in the specified directory. If the pathname specifies a
file then the server should send current information on the
file. A null argument implies the user's current working or
default directory. The data transfer is over the data
connection in type ASCII or type EBCDIC. (The user must
ensure that the TYPE is appropriately ASCII or EBCDIC).
Since the information on a file may vary widely from system
to system, this information may be hard to use automatically
in a program, but may be quite useful to a human user.
NAME LIST (NLST)
This command causes a directory listing to be sent from
server to user site. The pathname should specify a
directory or other system-specific file group descriptor; a
null argument implies the current directory. The server
will return a stream of names of files and no other
information. The data will be transferred in ASCII or
EBCDIC type over the data connection as valid pathname
strings separated by <CRLF> or <NL>. (Again the user must
ensure that the TYPE is correct.) This command is intended
to return information that can be used by a program to
further process the files automatically. For example, in
the implementation of a "multiple get" function.
SITE PARAMETERS (SITE)
This command is used by the server to provide services
specific to his system that are essential to file transfer
but not sufficiently universal to be included as commands in
the protocol. The nature of these services and the
specification of their syntax can be stated in a reply to
the HELP SITE command.
SYSTEM (SYST)
This command is used to find out the type of operating
system at the server. The reply shall have as its first
word one of the system names listed in the current version
of the Assigned Numbers document [4].
STATUS (STAT)
This command shall cause a status response to be sent over
the control connection in the form of a reply. The command
may be sent during a file transfer (along with the Telnet IP
and Synch signals--see the Section on FTP Commands) in which
case the server will respond with the status of the
operation in progress, or it may be sent between file
transfers. In the latter case, the command may have an
argument field. If the argument is a pathname, the command
is analogous to the "list" command except that data shall be
transferred over the control connection. If a partial
pathname is given, the server may respond with a list of
file names or attributes associated with that specification.
If no argument is given, the server should return general
status information about the server FTP process. This
should include current values of all transfer parameters and
the status of connections.
HELP (HELP)
This command shall cause the server to send helpful
information regarding its implementation status over the
control connection to the user. The command may take an
argument (e.g., any command name) and return more specific
information as a response. The reply is type 211 or 214.
It is suggested that HELP be allowed before entering a USER
command. The server may use this reply to specify
site-dependent parameters, e.g., in response to HELP SITE.
NOOP (NOOP)
This command does not affect any parameters or previously
entered commands. It specifies no action other than that the
server send an OK reply.
The File Transfer Protocol follows the specifications of the Telnet
protocol for all communications over the control connection. Since
the language used for Telnet communication may be a negotiated
option, all references in the next two sections will be to the
"Telnet language" and the corresponding "Telnet end-of-line code".
Currently, one may take these to mean NVT-ASCII and <CRLF>. No other
specifications of the Telnet protocol will be cited.
FTP commands are "Telnet strings" terminated by the "Telnet end of
line code". The command codes themselves are alphabetic characters
terminated by the character <SP> (Space) if parameters follow and
Telnet-EOL otherwise. The command codes and the semantics of
commands are described in this section; the detailed syntax of
commands is specified in the Section on Commands, the reply sequences
are discussed in the Section on Sequencing of Commands and Replies,
and scenarios illustrating the use of commands are provided in the
Section on Typical FTP Scenarios.
FTP commands may be partitioned as those specifying access-control
identifiers, data transfer parameters, or FTP service requests.
Certain commands (such as ABOR, STAT, QUIT) may be sent over the
control connection while a data transfer is in progress. Some
servers may not be able to monitor the control and data connections
simultaneously, in which case some special action will be necessary
to get the server's attention. The following ordered format is
tentatively recommended:
1. User system inserts the Telnet "Interrupt Process" (IP) signal
in the Telnet stream.
2. User system sends the Telnet "Synch" signal.
3. User system inserts the command (e.g., ABOR) in the Telnet
stream.
4. Server PI, after receiving "IP", scans the Telnet stream for
EXACTLY ONE FTP command.
(For other servers this may not be necessary but the actions listed
above should have no unusual effect.)
4.2. FTP REPLIES
Replies to File Transfer Protocol commands are devised to ensure
the synchronization of requests and actions in the process of file
transfer, and to guarantee that the user process always knows the
state of the Server. Every command must generate at least one
reply, although there may be more than one; in the latter case,
the multiple replies must be easily distinguished. In addition,
some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an
intermediate state if all preceding commands have been successful.
A failure at any point in the sequence necessitates the repetition
of the entire sequence from the beginning.
The details of the command-reply sequence are made explicit in
a set of state diagrams below.
An FTP reply consists of a three digit number (transmitted as
three alphanumeric characters) followed by some text. The number
is intended for use by automata to determine what state to enter
next; the text is intended for the human user. It is intended
that the three digits contain enough encoded information that the
user-process (the User-PI) will not need to examine the text and
may either discard it or pass it on to the user, as appropriate.
In particular, the text may be server-dependent, so there are
likely to be varying texts for each reply code.
A reply is defined to contain the 3-digit code, followed by Space
<SP>, followed by one line of text (where some maximum line length
has been specified), and terminated by the Telnet end-of-line
code. There will be cases however, where the text is longer than
a single line. In these cases the complete text must be bracketed
so the User-process knows when it may stop reading the reply (i.e.
stop processing input on the control connection) and go do other
things. This requires a special format on the first line to
indicate that more than one line is coming, and another on the
last line to designate it as the last. At least one of these must
contain the appropriate reply code to indicate the state of the
transaction. To satisfy all factions, it was decided that both
the first and last line codes should be the same.
Thus the format for multi-line replies is that the first line
will begin with the exact required reply code, followed
immediately by a Hyphen, "-" (also known as Minus), followed by
text. The last line will begin with the same code, followed
immediately by Space <SP>, optionally some text, and the Telnet
end-of-line code.
For example:
123-First line
Second line
234 A line beginning with numbers
123 The last line
The user-process then simply needs to search for the second
occurrence of the same reply code, followed by <SP> (Space), at
the beginning of a line, and ignore all intermediary lines. If
an intermediary line begins with a 3-digit number, the Server
must pad the front to avoid confusion.
This scheme allows standard system routines to be used for
reply information (such as for the STAT reply), with
"artificial" first and last lines tacked on. In rare cases
where these routines are able to generate three digits and a
Space at the beginning of any line, the beginning of each
text line should be offset by some neutral text, like Space.
This scheme assumes that multi-line replies may not be nested.
The three digits of the reply each have a special significance.
This is intended to allow a range of very simple to very
sophisticated responses by the user-process. The first digit
denotes whether the response is good, bad or incomplete.
(Referring to the state diagram), an unsophisticated user-process
will be able to determine its next action (proceed as planned,
redo, retrench, etc.) by simply examining this first digit. A
user-process that wants to know approximately what kind of error
occurred (e.g. file system error, command syntax error) may
examine the second digit, reserving the third digit for the finest
gradation of information (e.g., RNTO command without a preceding
RNFR).
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The requested action is being initiated; expect another
reply before proceeding with a new command. (The
user-process sending another command before the
completion reply would be in violation of protocol; but
server-FTP processes should queue any commands that
arrive while a preceding command is in progress.) This
type of reply can be used to indicate that the command
was accepted and the user-process may now pay attention
to the data connections, for implementations where
simultaneous monitoring is difficult. The server-FTP
process may send at most, one 1yz reply per command.
2yz Positive Completion reply
The requested action has been successfully completed. A
new request may be initiated.
3yz Positive Intermediate reply
The command has been accepted, but the requested action
is being held in abeyance, pending receipt of further
information. The user should send another command
specifying this information. This reply is used in
command sequence groups.
4yz Transient Negative Completion reply
The command was not accepted and the requested action did
not take place, but the error condition is temporary and
the action may be requested again. The user should
return to the beginning of the command sequence, if any.
It is difficult to assign a meaning to "transient",
particularly when two distinct sites (Server- and
User-processes) have to agree on the interpretation.
Each reply in the 4yz category might have a slightly
different time value, but the intent is that the
user-process is encouraged to try again. A rule of thumb
in determining if a reply fits into the 4yz or the 5yz
(Permanent Negative) category is that replies are 4yz if
the commands can be repeated without any change in
command form or in properties of the User or Server
(e.g., the command is spelled the same with the same
arguments used; the user does not change his file access
or user name; the server does not put up a new
implementation.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did
not take place. The User-process is discouraged from
repeating the exact request (in the same sequence). Even
some "permanent" error conditions can be corrected, so
the human user may want to direct his User-process to
reinitiate the command sequence by direct action at some
point in the future (e.g., after the spelling has been
changed, or the user has altered his directory status.)
The following function groupings are encoded in the second
digit:
x0z Syntax - These replies refer to syntax errors,
syntactically correct commands that don't fit any
functional category, unimplemented or superfluous
commands.
x1z Information - These are replies to requests for
information, such as status or help.
x2z Connections - Replies referring to the control and
data connections.
x3z Authentication and accounting - Replies for the login
process and accounting procedures.
x4z Unspecified as yet.
x5z File system - These replies indicate the status of the
Server file system vis-a-vis the requested transfer or
other file system action.
The third digit gives a finer gradation of meaning in each of
the function categories, specified by the second digit. The
list of replies below will illustrate this. Note that the text
associated with each reply is recommended, rather than
mandatory, and may even change according to the command with
which it is associated. The reply codes, on the other hand,
must strictly follow the specifications in the last section;
that is, Server implementations should not invent new codes for
situations that are only slightly different from the ones
described here, but rather should adapt codes already defined.
A command such as TYPE or ALLO whose successful execution
does not offer the user-process any new information will
cause a 200 reply to be returned. If the command is not
implemented by a particular Server-FTP process because it
has no relevance to that computer system, for example ALLO
at a TOPS20 site, a Positive Completion reply is still
desired so that the simple User-process knows it can proceed
with its course of action. A 202 reply is used in this case
with, for example, the reply text: "No storage allocation
necessary." If, on the other hand, the command requests a
non-site-specific action and is unimplemented, the response
is 502. A refinement of that is the 504 reply for a command
that is implemented, but that requests an unimplemented
parameter.
4.2.1 Reply Codes by Function Groups
200 Command okay.
500 Syntax error, command unrecognized.
This may include errors such as command line too long.
501 Syntax error in parameters or arguments.
202 Command not implemented, superfluous at this site.
502 Command not implemented.
503 Bad sequence of commands.
504 Command not implemented for that parameter.
110 Restart marker reply.
In this case, the text is exact and not left to the
particular implementation; it must read:
MARK yyyy = mmmm
Where yyyy is User-process data stream marker, and mmmm
server's equivalent marker (note the spaces between markers
and "=").
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.
On how to use the server or the meaning of a particular
non-standard command. This reply is useful only to the
human user.
215 NAME system type.
Where NAME is an official system name from the list in the
Assigned Numbers document.
120 Service ready in nnn minutes.
220 Service ready for new user.
221 Service closing control connection.
Logged out if appropriate.
421 Service not available, closing control connection.
This may be a reply to any command if the service knows it
must shut down.
125 Data connection already open; transfer starting.
225 Data connection open; no transfer in progress.
425 Can't open data connection.
226 Closing data connection.
Requested file action successful (for example, file
transfer or file abort).
426 Connection closed; transfer aborted.
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
230 User logged in, proceed.
530 Not logged in.
331 User name okay, need password.
332 Need account for login.
532 Need account for storing files.
150 File status okay; about to open data connection.
250 Requested file action okay, completed.
257 "PATHNAME" created.
350 Requested file action pending further information.
450 Requested file action not taken.
File unavailable (e.g., file busy).
550 Requested action not taken.
File unavailable (e.g., file not found, no access).
451 Requested action aborted. Local error in processing.
551 Requested action aborted. Page type unknown.
452 Requested action not taken.
Insufficient storage space in system.
552 Requested file action aborted.
Exceeded storage allocation (for current directory or
dataset).
553 Requested action not taken.
File name not allowed.
4.2.2 Numeric Order List of Reply Codes
110 Restart marker reply.
In this case, the text is exact and not left to the
particular implementation; it must read:
MARK yyyy = mmmm
Where yyyy is User-process data stream marker, and mmmm
server's equivalent marker (note the spaces between markers
and "=").
120 Service ready in nnn minutes.
125 Data connection already open; transfer starting.
150 File status okay; about to open data connection.
200 Command okay.
202 Command not implemented, superfluous at this site.
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.
On how to use the server or the meaning of a particular
non-standard command. This reply is useful only to the
human user.
215 NAME system type.
Where NAME is an official system name from the list in the
Assigned Numbers document.
220 Service ready for new user.
221 Service closing control connection.
Logged out if appropriate.
225 Data connection open; no transfer in progress.
226 Closing data connection.
Requested file action successful (for example, file
transfer or file abort).
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
230 User logged in, proceed.
250 Requested file action okay, completed.
257 "PATHNAME" created.
331 User name okay, need password.
332 Need account for login.
350 Requested file action pending further information.
421 Service not available, closing control connection.
This may be a reply to any command if the service knows it
must shut down.
425 Can't open data connection.
426 Connection closed; transfer aborted.
450 Requested file action not taken.
File unavailable (e.g., file busy).
451 Requested action aborted: local error in processing.
452 Requested action not taken.
Insufficient storage space in system.
500 Syntax error, command unrecognized.
This may include errors such as command line too long.
501 Syntax error in parameters or arguments.
502 Command not implemented.
503 Bad sequence of commands.
504 Command not implemented for that parameter.
530 Not logged in.
532 Need account for storing files.
550 Requested action not taken.
File unavailable (e.g., file not found, no access).
551 Requested action aborted: page type unknown.
552 Requested file action aborted.
Exceeded storage allocation (for current directory or
dataset).
553 Requested action not taken.
File name not allowed.
(page 43 continued on part 3)